It is well known for glass melting tanks to include a melting chamber in which solid batch material is heated to produce molten glass before entering a refining chamber in which the molten glass is at a sufficiently high temperature for refining to occur and thereby reduce defects due to impurities or bubble in the glass. Commonly the glass passes from a refining chamber through a conditioning zone in which thermal conditioning by controlled cooling is effected prior to glass leaving the tank through an outlet to a forming process. Such tanks may be used for continuous production of molten glass and are particularly applicable to the production of high quality glass for use in the production of flat glass.
When solely electric heating is used in a melting chamber of such a tank it is normal for the molten glass in the melting chamber to be covered by a cold top of solid batch material which is progressively melted by heat from electrodes immersed in the glass in the melting chamber. The flow path for molten glass from the melting chamber to a refining chamber, when using electric melting may be through a throat located adjacent the base of the melting chamber in order to reduce the probability of unmelted batch material being carried with the molten glass into the refining zone.
It is common in flame fired furnaces for the molten glass in a refining chamber to be sufficiently deep to permit recirculation of molten glass in convective flows so that the upper layers of glass in the refining zone are flowing towards a downstream end of the zone with a return flow in the lower region of the refining chamber.
It is known to provide a riser chamber after the melting chamber. It is also known to provide heating in such a riser chamber. However, serious problems can arise from unwanted corrosion of refractory walls of the riser chamber by the upward flowing glass, particularly where the riser chamber is increasing the temperature of the molten glass to a suitable refining temperature above that of the glass entering from the melting chamber, such as may be necessary in the production of high quality flat glass.
It is also known, for example from FIGS. 1 and 2 of U.S. Pat. No. 4,900,337, to use electrodes in a riser chamber forming a conditioning chamber following a throat from a melting chamber. However conditioning involves controlled cooling where electrodes are used to control the rate of loss of temperature of the molten glass rather than raising the temperature above that of the glass leaving the melting chamber. Where the temperature is not raised after passing through the throat into the riser the problems of corrosion in the riser are less severe due to the lower temperatures employed. Such arrangements where the glass temperature is not increased after leaving the melting chamber are more suitable for making glass for containers or fibre glass but may not provide the necessary refining for high quality flat glass production such as that used for a float glass production line.